The Warden is a ‘spare’ boss you face in Act I- curiously, you only face him once in each level of difficulty, not having to deal with him on a return trip on your own. While he marks an interesting point- the point where you begin your approach to the true boss of Act I, the Butcher, he doesn’t actually prepare you for a boss fight.
The Warden serves as excellent preparation for later common and champion enemies though.
The Warden is a giant skeletal man, wielding a tetsubo- for those who aren’t familiar with it, the tetsubo is a large club composed mainly of wood whose striking length is wrapped in iron that has spikes, rivets, or large lumps sticking out of it. As you might expect from something wielding a weapon like that, the Warden has hefty melee attacks that will inflict knockback, something common to enemies in act III and IV especially. He also has a simple projectile attack- a trio of ground-running fireballs.
Finally, the Warden is likely to be the first opponent you face who will ‘Imprison’ you. Periodically, while fighting the Warden, your character will be surrounded by an orange-yellow glow and become unable to leave the spot they stand on. While this is annoying, it is more of an inconvenience than an actual hindrance (unless you’re particularly frail and have no allies). Later in the game, many champion enemies and unique enemies will have such an ability, which can couple with other attacks to devastating effect- but the Warden and the common undead he summons don’t really have a good way to take advantage of it.
Aside from all these, the Warden is a decent-sized sack of health, and if you simply stand there and face him, you’re likely to get overwhelmed with his minions unless you’re packing some form of area attack.
With such an attack, though, and decent defenses, the Warden is nothing to scream about. He does provide an interesting fight while you face him, and serves to break up the dungeon crawl that the latter part of Act I is composed of- and for that he deserves to be appreciated.
Like a speedbump really- mildly annoying sometimes, mildly helpful others- but overall fairly useful and nice to have around.